Some evidence supports the hypothesis that the mutation rate at a particular site is influenced by neighboring base-pairs. I have sought to modify the reversion and conversion frequencies of bacteriophage T4 rII nonsense codons by inserting nearby temperature-sensitive rII lesions. The insertion of a ts mutation reduces the 2-aminopurine-induced reversion of an rII amber mutation about three-fold. The ts mutation reduces the corresponding UAA-UGA conversion about eight-fold, while the reversion of the ochre codon to glutamine (UAA arrow to the right CAA) is not affected. Selection controls show no measurable selection against the ts marker. DNA sequence analysis suggests that the rate-modulation effect of the ts mutation may occur because of a change in DNA secondary structure.